In a conventional horizontal GaN-based high electron mobility transistor (HEMT) device with a two-dimensional electron gas (2DEG), the device cannot be normally off due to the 2DEG distribution of the gate region. It has been proposed that the 2DEG distribution of the gate region is destroyed to enhance the threshold voltage (Vt), but such method may lower the output current. It also has been proposed that a p-type AlGaN layer is formed in the gate region by a selectivity area growth. Using the characteristic of the depletion region, the 2DEG distribution below the gate region is depleted so that the device can be normally off. The turn-on current of such device is not much affected, but the threshold voltage of the same is generally lower because the threshold voltage (Vth) is limited by whether the p-type carrier concentration (about 1×1017/cm2) of the p-type AlGaN layer can effectively deplete the 2DEG. Therefore, attention is drawn to an enhancement mode hetero-structure field effect transistor (HFET) device in which the threshold voltage is improved but the turn-on current is not much affected.